The analysis of liquid samples, more specifically blood samples, is frequently carried out in the diagnosis of various medical conditions. In particular, processing a sample on a microfluidic liquid handling structure, for example provided on a cartridge, is a widely-used technique. Such processing may include centrifugally driving liquid flow within the cartridge by rotating it.
Processing a blood sample may also include imaging the sample and the cells contained within it. Imaging the sample in this way can be used to count blood cells and the number of different types of blood cell. Generally, only a drop of blood is used in the processing and the dimensions of the microfluidic liquid handling structures of the cartridge are of the order of micrometers. For example, a detection zone in which a blood sample is held during the imaging may be 500 μm across. In order to take images of a sample when it is in such a detection zone, the detection zone must be aligned with an image capture device. The field of view of the image capture device may typically be 200 μm across and to align the detection zone and the field of view of the image capture device, a sufficient degree of accuracy in angularly positioning the detection zone of the cartridge relative to the image capture device is required.
As mentioned above, processing a blood sample may also involve centrifugally driving liquid flow within the cartridge by rotating the cartridge. To centrifugally drive liquid flow, the cartridge must be rotated at frequencies higher than those that would generally be used to position a cartridge relative to an image capture device. To rotate the cartridge at these higher frequencies, a motor such as those typically used to spin a disc in a DVD or CD drive, such as a brushless DC (BLDC) motor, may be used. An advantage of using this type of motor is that such motors are widely available and inexpensive, compared to more specialised motors often used in laboratory experiments. However, whilst such motors are suitable for rotation at frequencies required to centrifugally drive liquid flow, they do not have sufficient accuracy, in terms of angular positioning of the cartridge, to align a detection zone of a cartridge, which may be 500 μm across, with an image capture device with a field of view that may be 200 μm across. A typical motor such as those typically used to spin a disc in a DVD or CD drive, for example a brushless DC motor, is not sufficiently accurate for the purposes of aligning a detection zone and an image capture device such as those described above. Motors may be available which would be capable of both rotating the cartridge at frequencies sufficiently high in order to centrifugally drive liquid flow in the cartridge and aligning the detection zone with the image capture device, but such motors are likely to be more expensive than motors such as those used in DVD or CD drives.
It would be desirable for a drive system for an analytic cartridge to be able to accurately position the cartridge and drive it at a sufficient speed in a way that affords significant design freedom and flexibility and/or in a cost efficient manner.